The present invention relates to lubricant compositions for motorcycle engines where the crankcase lubricant is also used to lubricate a wet clutch.
Lubricants for motorcycles typically provide lubrication for the engine (a crankcase) and a wet clutch. These two devices, although often lubricated by the same fluid, often have different lubrication requirements. For example, the lubrication of the engine desirably provides low metal-on-metal friction, to promote good fuel economy. (Typically, the “metal” referred to is steel.) However, the friction coefficient for the metal-on-composition interfaces located within the wet clutch is typically desired to be relatively high, to assure good engagement and power transmission. Additionally, motorcycle lubricants will also lubricate other devices such as gears or bearings, each having their own lubricating requirement. Many lubricants have been designed over the year for lubrication of motorcycles (also known as motorbikes or motorscooters). One such lubricant is described in U.S. Patent Publication 2008-0096778, Breon et al., Apr. 24, 2008.
Because of the varied and demanding lubrication performance required of them, motorcycle lubricants are typically designed specifically for use in motorcycles. That is, typical lubricants as used in lubricating passenger car engines are not normally used for motorcycles. Such lubricants may exhibit a low coefficient of friction that is undesirable for lubricating the wet clutch found in most motorcycles. The two types of lubricant technologies have, simply put, diverged in recent years.
Various friction-reducing additives are known. Glycerol monooleate (“GMO”) is a well-known friction modifier for engines as disclosed in, e.g., U.S. Patent Publication 2008-0280795, Fujitsu, Nov. 13, 2008. However, GMO does not appear to be particularly effective in the present application. Various molybdenum compounds are also known as friction modifiers, as disclosed in, the aforementioned US 2008-0280795. However, Molybdenum dithiocarbamate compounds, while particularly effective at reducing dynamic friction in an internal combustion engine can present challenges when used in motorcycle wet clutch applications.
The disclosed technology, therefore, solves the problem of providing improved fuel economy and antioxidancy while maintaining clutch control in motorcycles equipped with a wet clutch. This is accomplished by supplying to both the crankcase and the clutch a lubricating composition comprising an oil of lubricating viscosity, an antimony dithiocarbamate compound, and an ash-free friction modifier. The disclosed technology may also be used to optimize power and acceleration while reducing oil sump temperatures.